SEOUL -- North Korea has rebuilt facilities for rocket launches and engine testing at its space center, U.S. monitors said, based on fresh satellite imagery. President Donald Trump expressed disappointment in his carefully worded comment.

"A little disappointed. Little bit," Trump said during a meeting with the Czech prime minister at the White House. "We'll see. We'll let you know in about a year."

Satellite imagery from March 6 showed that work on restoring the launch pad and the engine test stand at the Sohae space center in Tongchang-ri has continued" at a rapid pace," according to 38 North, a website which monitors North Korea's missile and nuclear activities.

"Given that construction plus activity at other areas of the site, Sohae appears to have returned to normal operational status," the website said.

Work on the rail-mounted transfer structure at the launch pad appears to have been completed by March 6 and it may now be operational, while progress has been made on rebuilding the support structure at the engine test stand, 38 North said.

The Center for Strategic and International Studies (CSIS), a think tank, gave similar analysis, saying the vertical engine test stand superstructure, along with its added environmental covering, has been rebuilt.

Additional work such as the construction of a new environmental shelter on the entrance ramp could indicate "deliberate preparations" to test rocket engines again, CSIS said, adding the rail-mounted transfer structure has been rebuilt and moved back to its normal location.

"These activities are consistent with an unwinding or 'snapback' of the modest disablement measures undertaken by the North Koreans after the Singapore Summit in June 2018, and speak to the ease with which the DPRK (North Korea) can reverse steps it might take toward denuclearization in the future," CSIS said.

At an inter-Korean summit in September last year, North Korean leader Kim Jong-un pledged to permanently dismantle facilities for ballistic missile launches and engine testing in Sohae in the presence of international inspectors.

U.S. and North Korean leaders failed to strike a deal at their summit in Vietnam, triggering debate over whether Pyongyang is going backward. However, U.S. National Security Adviser John Bolton said Trump is open to talking to North Korea again.

"President's obviously open to talking again," Bolton said in an interview on Fox News. "We'll see when that might be scheduled or how it would work out. But he thinks the deal is there if North Korea is prepared to look at the big picture."

"I think the North Koreans obviously would like to give up as little of their nuclear, biological and chemical weapons and ballistic missile programs as they could in exchange for very broad sanctions relief, and that's basically what they asked for," Bolton said. "President wasn't buying it."